Metal can being a pressure tight metal packaging

ABSTRACT

Metal can wall suitable to form part of a pressure tight metal packaging further comprising a bottom and a top adjoining the wall having essentially one main wall thickness, whereby the wall is provided with means other than the bottom or the top supporting the wall essentially along its circumference.

[0001] The invention relates to a metal can, being a pressure tightmetal packaging such as a two piece steel light gauge aerosol can,comprising an ironed side wall stretching essentially along an axialdirection and a circumferential direction, and a first end closureadjoining the side wall in a first wall-to-closure transition portion,and a second end closure adjoining the side wall in a secondwall-to-closure transition portion, which side wall comprises a mainside wall portion stretching from the first wall-to-closure transitionportion to the second wall-to-closure transition portion, wherein themain side wall portion is provided with means of increased side wallthickness supporting the side wall essentially along the circumferencethe can, having superior vacuum resistance.

[0002] It is common practice in a metal can that the thickness of theside wall in its wall-to-closure transition portions is greater than thethickness in the main side wall portion.

[0003] There is a constant strive to down gauge the metal sheet fromwhich metal cans are manufactured in general and the steel sheet usetherefor in particular, in order to save material and to reduce “dead”weight in the distribution chain. However, customers of can makers, suchas the fillers of the cans, are reluctant to reduce the wall thicknessof aerosol cans because of the suspected lower vacuum performance ofsuch a down gauged can. A lower vacuum performance increases the risk ofcollapsed cans during filling in case of dented cans.

[0004] A metal can with improved vacuum performance is known. Americanpatent U.S. Pat. No. 3,951,296 discloses a wall-ironed container havinga plurality of reinforcing ribs being spaced from the ends of the sidewalls and from each other, in order to achieve increased resistanceagainst buckling of the side wall and having improved resistance againstvacuum. In axial cross section, the ribs are trapezium shaped, with acentral surface parallel to the inside surface of the side walls havingan axial length of 2,36 mm, and both wedging surfaces inclined at anangle of 24° from the inside surface of the side walls to the centralsurface of the reinforcing ribs. The ribs protrude inwards over adistance of 167 μm. The known ribs are formed in the side wall by meansof ironing the side wall in an ironing ring against a punch in whichgrooves have been formed spaced from one another along the longitudinalaxis of the punch. In an ironing operation, the side wall is passedthrough the ironing ring by which the side wall is reduced in thicknessand elongated.

[0005] The known can has satisfactory vacuum performance, but it isdifficult to fabricate in a plural-ring ironing process. As the sidewall elongates under the action of a second ironing ring, thereinforcing ribs move out of the respective grooves in which they wereformed. Their destruction has to be prevented.

[0006] According to the present invention, it is an objective to providea novel can concept wherein considerable material and weight savings, inthe order of 5% and even more, can be achieved without unacceptablyaffecting the performance, notably the vacuum performance.

[0007] According to the invention a metal can, being a pressure tightmetal packaging, is provided, comprising an ironed side wall stretchingessentially along an axial direction and a circumferential direction,and a first end closure adjoining the side wall in a firstwall-to-closure transition portion, and a second end closure adjoiningthe side wall in a second wall-to-closure transition portion, which sidewall comprises a main side wall portion stretching from the firstwall-to-closure transition portion to the second wall-to-closuretransition portion, wherein the main side wall portion is provided withmeans of increased side wall thickness supporting the side wallessentially along the circumference of the can, wherein the meanssupporting the side wall consist of one annular portion in which thethickness of the side wall is greater than the side wall thickness inthe main side wall portion outside the annular portion.

[0008] For the purpose of the application, the annular portion isconsidered to comprise also an annular portion in which substantialregions along the circumference have a side wall thickness that isgreater than the side wall thickness in the main side wall portionoutside the annular portion.

[0009] Surprisingly the vacuum performance can be greatly enhanced bysupporting the can wall only locally along its circumference byproviding only one supporting annular portion, for instance areinforcement rib, which forms part of the wall and is thus integratedtherein. A side wall with only one reinforcement rib can beadvantageously fabricated using multiple ironing steps in a plural-ringironing process, since a formed rib will not be destructed by followingironing steps.

[0010] It is remarked that a semi-product is known from European patentEP B 0 122 651, which semi-product comprises a can side wall which has agreater wall thickness in one annular centre portion than in the otherportions of the side wall. To obtain two end products, this semi-productis first cut transversely across the annular centre region, before it isprovided with a closure to form a metal can having a side wall with agreater wall thickness adjacent the closures than at other regions. Inthe end-products, the greater wall thickness is localised in thewall-to-closure transition portions of the can walls, for the formationof a flanged connection with the closure. The thus obtained cans do nothave a circumferential rib in the main side wall portion of their walls.

[0011] In an embodiment of the invention, the annular portion isintersected along essentially the entire circumference by a crosssectional plane through the metal can, located halfway between the firstwall-to-closure transition portion and the second wall-to-closuretransition portion. Such a located rib provides the best vacuumperformance of the metal can.

[0012] In a suitable embodiment, the annular portion is acircumferential rib protruding inwardly from the inside surface of theside wall into the packaging. This has the advantage that the outsidesurface of the packaging is undisturbed.

[0013] In an embodiment of the invention, the side wall thickness in theannular portion is not more than 40 μm thicker than the side wallthickness in the remainder of the main side wall portion. When theannular portion extends more than 40 μm from the main side wall of thepackaging, it becomes problematic to strip the side wall from a wallironing tool. For instance, stripping from a wall ironing punch becomesproblematic in the case the annular portion protrudes into the packagingby more than 40 μm.

[0014] By preference, the side wall thickness is not more than 30 μmthicker than the side wall thickness in the remainder of the main sidewall portion. Herewith the stripping problems are limited to a moreacceptable level.

[0015] More by preference, the side wall thickness is not more than 20μm thicker than the side wall thickness in the remainder of the mainside wall portion. The stripping behaviour of a side wall with an inwardprotrusion of 20 μm has been found to be approximately equal to that ofa straight wall can.

[0016] In an embodiment of the invention, the annular portion comprises,when seen in a longitudinal section of the metal can, a portion whereinthe side wall thickness is constant over an axial distance. Thisprovides a further advantage in fabrication, in that such a metal canside wall is better stripped from a wall ironing tool, for instance awall ironing punch.

[0017] Additionally, in the case that the first end closure is integralto the side wall, the thickness of the side wall in the annular portionof the main side wall portion, when seen in a longitudinal section ofthe metal can and measured at increasing distances from the first endclosure, first gradually increases from the thickness of the side walloutside the annular portion to a maximum thickness of the side wallinside the annular portion over a section with an axial length D₁, andthen decreases from the maximum thickness to the thickness in theremainder of the main side wall portion outside the annular portion overa section with an axial length D₂, which length is shorter than D₁. Thisis done with regard to the stripping direction of the metal can sidewall, and further improves the stripping behaviour from the wall ironingpunch.

[0018] In an embodiment wherein the first end closure is integral to theside wall, it is advantageous that, when seen in a longitudinal sectionof the metal can, the thickness of the side wall in a section of theannular portion of the main side wall portion, measured at increasingdistances from the first end closure, gradually increases from thethickness of the side wall outside the annular portion to a maximumthickness of the side wall inside the annular portion, in which sectionthe surface of the side wall inside the packaging is wedged with respectto the corresponding surface of the side wall on the outside of thepackaging at an angle between 0.01 and 5°. The lower limit of the rangeof angles is related to the available space between the annular portionand a wall-to-closure transition portion. The upper limit marks an angleabove which it becomes increasingly problematic to strip the side wallfrom a wall ironing tool.

[0019] By preference, this surface is wedged at an angle between 0.01and 1°. Herewith, stripping problems are even better avoided.

[0020] More by preference, this surface is wedged at an angle between0.01 and 0.25°. Herewith stripping behaviour can be approximately equalto that of a straight wall can.

[0021] Further, a can wall according to the invention may bemanufactured by conventional drawing and wall ironing processes.

[0022] The invention is also embodied in a method of forming a metal canwall according to the invention, wherein on the side of the wall facingthe inside of the packaging the wall during forming is supported by apunch and on its other side it is during forming brought into contactwith a forming die, characterised in that a profiled punch is usedhaving a cavity in its working surface that corresponds with theprotrusion.

[0023] Finally the invention is also embodied in a punch having a cavityaccommodating and causing when in use the protrusion in the wall portionaccording to the invention in the method according to the invention.

[0024] The invention will be illustrated in the following in moredetail, also using the drawings wherein:

[0025]FIG. 1 shows a longitudinal section partly in perspective of a canbody showing an inwardly protruding thicker wall portion;

[0026]FIG. 2 shows a longitudinal section through the centre line of acan body showing an inwardly protruding thicker wall portion indicatingdimension symbols;

[0027]FIG. 3 shows the actual dimensions of several cans tested;

[0028]FIG. 4 shows the relation between applied vacuum and collapsingforce for different can types;

[0029]FIG. 5 shows the dimensions of a carbide punch used in the methodaccording to the invention;

[0030]FIG. 6 shows the dimensions of a steel punch used in the methodaccording to the invention.

[0031] For investigations three types of cans were produced on acommercially available bodymaker:

[0032] A first type of a known can concept with a straight wall with awall thickness of 0.15 mm.

[0033] A second type of a can according to the invention with a wallthickness of 0.15 mm and a mid wall step of 0.02 mm×40 mm, i.e. withH1=44 mm, H2=52 mm, H3=92 mm, H4=96 mm and X1=0.170 mm, see FIG. 2;

[0034] A third type of a can according to the invention with a wallthickness of 0.15 mm and a mid wall step of 0.03 mm×20 mm, i.e. withH1=50 mm, H2=62 mm, H3=82 mm, H4=88 mm and X1=0.180 mm, see FIG. 2.

[0035] A fourth type of can commercially available in the market havinga straight wall with a wall thickness of 0.167 mm was used as areference.

[0036] For the test cans and the reference cans T57 packaging steelgrade was used. This is a regular material commercially supplied to themarket to manufacture such cans.

[0037] Five hundred trial cups were made in two stages: The first drawwas done on a separate cupping press.

[0038] The second draw was carried out on a separate cupping press tothe final diameter of 45 mm.

[0039] For a can of the first type a punch was used with diameter 44.917mm. This punch was reground afterwards for a can of the second type witha step of 0.02 mm×40 mm while changing the nominal diameter to 44.913mm.

[0040] A new tool steel punch was manufactured for a can of the thirdtype with a step of 0.03 mm×20 mm.

[0041] The cans of the second type and the third type were provided withan inwardly protruding thicker wall portion. This has the advantage thatthe outside surface of the packaging is undisturbed.

[0042] The transition zone length in front and at the end of the punchstep was chosen to be different. This is done with regard to thestripping direction of the cans. TABLE 1 shows the used ironing dies:

1^(st) die

2^(nd) die

3^(rd) die

4^(th) die [mm] [mm] [mm] [mm] 0.15 cans 45.56 45.33 45.20 45.21 0.15“step” cans 45.54 45.32 45.22 45.21

[0043] All cans were wall ironed on one and the same body maker.

[0044] The stripping behaviour of the test cans of the second type withX1=0.170 mm was approximately equal to that of the straight wall cans.Some stripping problems occurred with cans of the third type with aX1=0.180 mm. The reason for this is the difference in step size and theuse of a tool steel punch, which gives a higher friction.

[0045] A profile of the wall thickness of the different can types wasmade with a thickness scanner. The wall thickness profile of the canswith a step was also measured after the third die in order to check theeffect of moving the thick wall by reducing the can in the fourth die,see FIG. 3 and table 2. TABLE 2 Measured thickness Thickness thickThickness thin wall [μm] wall [μm] 0.17 reference cans 167 167 0.15reference cans 152 152 0.15, step 0.02 cans 3^(rd) die 188 167 0.15,step 0.03 cans 3^(rd) die 205 175 0.15, step 0.02 cans 172 150 0.15,step 0.03 cans 183 155

[0046] From FIG. 3 and table 2 it can be concluded that the wallthickness between the different can types is approximately equal. Thethickness of the step is 22 and 28 microns respectively. There is nomeasurable material displacement in the step, caused by the fourth dieoperation.

[0047] Each type of can was tested on vacuum performance using aforce-gauge. A force perpendicular to the centre line of the can,hereafter called the T-bar force, was applied on the wall of the can inthe middle of the can height, i.e. where the test can according to theinvention had a local annular wall portion with a greater thickness. Thecan was de-pressurised to different vacuum levels. For each vacuumlevel, ten cans were tested by increasing the T-bar force. In FIG. 5 theresults are summarised.

[0048] Each point in FIG. 5 represents an average of 10 measurements.Some cans from a can of the second and third type were tested byapplying the force at a quarter of the can height, where the wall wasthin.

[0049] From the results it is concluded that the vacuum performance ofthe second can type according to the invention is approximately equal tothe performance of the reference can. That means that a local wallthickness increase according to the invention does increase the vacuumperformance. Further, it is clear that the length of the thick wall alsoplays a role in increasing the vacuum performance.

[0050] According to the invention it is now feasible to down gaugepackaging steel for aerosol type cans without losing vacuum performanceby providing the can wall with supporting means such as a supportingthicker portion along its circumference as dislosed herein.

1. Metal can, being a pressure tight metal packaging, comprising anironed side wall stretching essentially along an axial direction and acircumferential direction, and a first end closure adjoining the sidewall in a first wall-to-closure transition portion, and a second endclosure adjoining the side wall in a second wall-to-closure transitionportion, which side wall comprises a main side wall portion stretchingfrom the first wall-to-closure transition portion to the secondwall-to-closure transition portion, wherein the main side wall portionis provided with means of increased side wall thickness supporting theside wall essentially along the circumference of the can, consisting ofone annular portion of wall ironed metal in which the thickness of theside wall is greater than the side wall thickness in the main side wallportion outside the annular portion, characterised in that the oneannular portion comprises wall ironed metal that has been reduced inthickness by more than 39.4%, and the side wall thickness in the annularportion is less than 40 μm thicker than the side wall thickness in theremainder of the main side wall portion.
 2. Metal can according to claim1, characterised in that the one annular portion comprises wall ironedmetal that has been reduced in thickness by more than 41.6%, preferablyby more than 45.1%.
 3. Metal can according to claim 1 or 2,characterised in that the main side wall portion outside the annularportion comprises wall ironed metal that has been reduced in thicknessby more than 50.6%.
 4. Metal can according to claim 1, 2, or 3,characterised in that the main side wall portion comprises steel,preferably packaging steel.
 5. Metal can according to claim 1,characterised in that the annular portion is intersected alongessentially the entire circumference by a cross sectional plane throughthe metal can, located halfway between the first wall-to-closuretransition portion and the second wall-to-closure transition portion. 6.Metal can according to claim 1 or 5, characterized in that the annularportion is a circumferential rib protruding inwardly from the insidesurface of the side wall into the packaging.
 7. Metal can according toany one of the preceding claims, characterised in that the side wallthickness in the annular portion is not more than 28 μm thicker than, bypreference not more than 22 μm thicker, than the side wall thickness inthe remainder of the main side wall portion.
 8. Metal can according toany one of the preceding claims, characterised in that, when seen in alongitudinal section of the metal can, the annular portion comprises aportion wherein the side wall thickness is constant over an axialdistance.
 9. Metal can according to any one of the preceding claims,characterised in that, when seen in a longitudinal section of the metalcan, the thickness of the side wall in the annular portion of the mainside wall portion, measured at increasing distances from the first endclosure, first gradually increases from the thickness of the side walloutside the annular portion to a maximum thickness of the side wallinside the annular portion over a section with an axial length D₁, andthen decreases from the maximum thickness to the thickness in theremainder of the main side wall portion outside the annular portion overa section with an axial length D₂, which length is shorter than D₁, andwherein the first end closure is integral to the side wall.
 10. Metalcan according to any one of the preceding claims, characterised in that,the first end closure is integral to the side wall, and, when seen in alongitudinal section of the metal can, the thickness of the side wall ina section of the annular portion of the main side wall portion, measuredat increasing distances from the first end closure, gradually increasesfrom the thickness of the side wall outside the annular portion to amaximum thickness of the side wall inside the annular portion, in whichsection the surface of the side wall inside the packaging is wedged withrespect to the corresponding surface of the side wall on the outside ofthe packaging at an angle between 0.01 and 5°, by preference at an anglebetween 0.01 and 1°, more by preference at an angle between 0.01 and0.25°.
 11. Method of producing a metal can, being a pressure tight metalpackaging, comprising an ironed side wall stretching essentially alongan axial direction and a circumferential direction, and a first endclosure adjoining die side wall in a first wall-to-closure transitionportion, and a second end closure adjoining the side wall in a secondwall-to-closure transition portion, which side wall comprises a mainside wall portion stretching from the first wall-to-closure transitionportion to the second wall-to-closure transition portion, wherein themain side wall portion is provided with means of increased side wallthickness supporting the side wall essentially along the circumferenceof the can, consisting of one annular portion of wall ironed metal inwhich the thickness of the side wall is greater than the side wallthickness in the main side wall portion outside the annular portion, themethod comprising the steps of (i) producing cups in at least onedrawing operation, (ii) reducing the side wall thickness using at leastone ironing die and a punch having a forming contour in longitudinalcross section comprising an essentially straight section provided with arecess, the recess being provided for forming the annular portion,characterised in that the recess has a depth of less than 40 μm andthickness of the main side wall portion where the recess is located isreduced from at least 313.5 μm to less than 190 μm.